Frequency multiplying device for induction heating applications

ABSTRACT

There is disclosed a frequency multiplying device for induction heating applications in which the supply voltage is applied between two central taps of a heating inductor and a saturable reactor. The extremities of the saturable reactor are connected to those of a resonant circuit constituted by the heating inductor and a condenser in parallel therewith, through impedances such that the currents at supply frequency traversing them are of equal value and of symmetrical phases with respect to the phase of the supply voltage, the angle of each such phase being equal to 90* divided by the selected frequency multiplier and multiplied by an odd integer, so that these currents are in phase operation and their sum is nil.

[75] Inventor:

United States Patent 1 Trine FREQU ENCY MULTIPLYING DEVICE FOR INDUCTION HEATING APPLICATIONS 30] Foreign Application Priority Data Dec. 10, 1971 Belgium 3632 [52] US. Cl 2l9/10.75, 13/26, 321/68 [51] Int. Cl. H05b 5/06 [58] Field of Search 219/10.75, 10.77, 219/10.79, 10.43; 13/1, 26; 321/68, 69, 60, 7

[56] References Cited UNITED STATES PATENTS 1,698,284 1/1929 Van Aavco et aL... 321/68 X 3,055,959 9/1962 Vaughn 13/26 51 Dec. 4, 1973 Primary Examiner-Bruce A. Reynolds Attorney-Raymond A. Robic [5 7 ABSTRACT There is disclosed a frequency multiplying device for induction heating applications in which the supply voltage is applied between two central taps of a heating inductor and a saturable reactor. The extremities of the saturable reactor are connected to those of a resonant circuit constituted by the heating inductor and a condenser in parallel therewith, through impedances such that the currents at supply frequency traversing them are of equal value and of symmetrical phases with respect to the phase of the supply voltage, the angle of each such phase being equal to 90 divided by the selected frequency multiplier and multiplied by an odd integer, so that these currents are in phase operation and their sum is nil.

3 Claims, 2 Drawing Figures PATENTEB M975 FIG.2

FREQUENCY MULTIPLYING DEVICE FOR INDUCTION HEATING APPLICATIONS The present invention relates to frequency multiplying devices comprising a heating inductor forming, with a condenser in parallel, a resonant circuit supplied from a source of mono-phase voltage through the intermediary of a reactor which becomes saturated when the instantaneous voltage at its limits goes beyond a certain level, this resonant circuit being tuned to an odd multiple of the frequency of the supply voltage.

Known devices of this type present serious drawbacks; in particular the flow of currents of multiplied frequency in the supply source produces undesirable heating and poor yield and difficulties in starting the installation The frequency multiplying device according to the invention is free of these problems; due to symmetrical disposition, the multiple frequency currents flow in the two circuits in parallel practically at phase opposition in such a way that the resulting current which flows in the supply source is practically nil. This arrangement also produces few disturbances in the resonance of the resonant heating circuit when switching-in takes place so that the starting of the installation occurs without bumps.

The frequency multiplying device according to the invention is characterized in that the heating inductor and the saturable reactor are each provided with a center tap or at least one of them is constituted by two identical portions without mutual induction, the supply voltage being applied between these two taps and in that the extremities of the saturable reactor are connected to those of the resonant heating circuit respectively through impedances such that the currents at supply voltage which traverse them-are of equal value and of phases symmetrical with the phase of the supply voltage, the elements of the device being furthermore selected in such a way that the angle of each of these phases is equal to 90 divided by the selected frequency multiplier and multiplied by an odd integer.

The description hereinafter and the annexed drawing refer to an embodiment of a frequency multiplying device according to the invention given by way of example.

IN THE DRAWING FIG. 1 is a diagram of the circuit of a frequency multiplying device according to the invention, and

FIG. 2 is a vector diagram of the currents present in the circuit.

FIG. 1 represents an induction heating installation comprising an inductor L having a center tap and connected in parallel with a condenser C. Resistances r represent the active part of the impedance of the inductor. The resonant circuit LC is connected to the extremities of a saturable reactor S having a central tap, respectively through a condenser CS and a linear reactor LS; a voltage V of frequency f is applied between the central taps of reactor S and inductor L. This voltage is supplied by an alternator A through the intermediary of an autotransformer T with adjustable taps. The oscillating circuit LC is tuned to a frequency nf, n being an odd integer.

At each alternation of thefrequency voltage f at the limits of the reactor S,- thelatter becomes saturated starting from a certain instantaneous value, its impedance falls transitorily and a generation of harmonics results which excite the oscillation of the circuit LC as occurs in the known devices.

The condenser CS and the reactor LS are selected in such a way that the two circuits containing these two respective elements and which derive between the central taps of the reactor S and of the inductor L have reactances of equal value X and of opposite signs for the supply frequency f. These two circuits on the other hand are of equal resistance R so that if the currents of frequency fin the condenser CS, in the reactor LS and in the voltage supply source V are respectively designated as I,, I, and I, these currents may be represented by the vector diagram of FIG. 2. As can be seen therein, the current I which is the sum of I, and I, is in phase with the voltage V and the currents I, and I, of equal value are symmetrical with respect to the voltage V and form therewith each an angle 0:. These currents are thus out of phase with respect to each other by an angle 2a. On the other hand, the multiplied frequency currents nf which flow in the same circuits are out of phase with each other by an angle n times greater than between I, and 1, namely by an angle 2 na. If the device is arranged in such a way that 2 not Nx 180, N being an odd integer, the currents of frequency nf in the two derivation circuits will be in phase opposition and as a consequence, their sum will be nil. Thus there will not be any currents of frequency nf in the supply source.

The above condition is equivalent to saying that OF (N X 1,80/2 n) (N X /n) For example, in the case of a frequency quintuplying device (n 5) by giving to N the successive values 1, 3, 5, the angles obtained will be:

and so on.

The elements of the circuits are thus chosen in such a way that the angle a will have one of these values.

Furthermore, the highest rate of harmonics of order n is obtained by acting onthe voltage at the limits of the reactor S in such away as to obtain the optimum saturation conditions, which consists of acting on the voltage V by displacing the adjustable tap of the autotransformer T or by adjusting the excitation of alternator A.

It is evident that the reactor S or the inductor L or both may be constituted by two identical portions in series without mutual induction, the junction point of these two portions playing the part of the central tap.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Frequency multiplying device for induction heating applications comprising a heating inductor forming, with a condenser in parallel therewith, a resonant circuit supplied from a monophase voltage source through the intermediary of a saturable reactor which becomes saturated when the instantaneous voltage at its limits goes beyond a certain level, said resonant circuit being tuned to an odd multiple of the frequency of the supply voltage, characterized in that the heating inductor and the saturable reactor each comprise two identical portions without mutual induction with a junction point between said two portions, the supply voltage being ap- 2. Frequency multiplying device according to claim 1, characterized in that the heating inductor and the saturable reactor each comprise a central tap and the supply voltage is applied between said two taps.

3. Frequency multiplying device according to claim 1, characterized in that adjustable means for adjusting the supply voltage are provided to adjust the conditions of saturation of the saturable reactor to obtain the highest rate of harmonics desired. 

1. Frequency multiplying device for induction heating applications comprising a heating inductor forming, with a condenser in parallel therewith, a resonant circuit supplied from a monophase voltage source through the intermediary of a saturable reactor which becomes saturated when the instantaneous voltage at its limits goes beyond a certain level, said resonant circuit being tuned to an odd multiple of the frequency of the supply voltage, characterized in that the heating inductor and the saturable reactor each comprise two identical portions without mutual induction with a junction point between said two portions, the supply voltage being applied beTween said two junction points, and in that the extremities of the saturable reactor are connected to the extremities of the resonant heating circuit respectively through impedances selected so that the currents at supply frequency which traverse them are substantially of equal value and of phases symmetrical with respect to those of the supply voltage, the elements of the device being selected so that the angle of each of these phases is substantially equal to 90 divided by the selected frequency multiplier and multiplied by an odd integer.
 2. Frequency multiplying device according to claim 1, characterized in that the heating inductor and the saturable reactor each comprise a central tap and the supply voltage is applied between said two taps.
 3. Frequency multiplying device according to claim 1, characterized in that adjustable means for adjusting the supply voltage are provided to adjust the conditions of saturation of the saturable reactor to obtain the highest rate of harmonics desired. 